Two distinct populations of peptidergic (POMC and NPY/AgRP) neurons in the arcuate nucleus of the hypothalamus respond to metabolic cues such as leptin. Studies done in the previous grant cycle helped define the neuroanatomical targets of these leptin-responsive neurons including sympathetic preganglionic neurons and neurons in the lateral hypothalamic area (LHA) and paraventricular hypothalamic nucleus. Since the previous submission, another critical metabolic hormone, ghrelin, has been identified to regulate food intake and body weight. Evidence suggests that CMS neuronal groups are targets of ghrelin. This includes NPY/AgRP neurons in the arcuate nucleus. This project will employ state of the art functional neuroanatomical techniques including tract tracing, ghrelin induced gene expression using in situ hybridization histochemistry, and chemical identification of neuronal phenotypes (alone and in combination) to identify the anatomical projections of ghrelin-responsive neurons. We will determine to what degree populations of ghrelin responsive neurons in the arcuate and retrochiasmatic neurons expressing NPY/AGRP and POMC/CART project to sites including spinal cord, dorsal motor nucleus of the vagus, the lateral hypothalamic area and the paraventricular nucleus, which are likely to play a distinct role in activating the efferent pathways activated by ghrelin. Finally, we will directly test the hypothesis that ghrelin receptor expression only in NPY/AgRP neurons is sufficient to mediate the effects of ghrelin to increase food intake and decrease energy expenditure. To accomplish this we will use a novel mouse model that is null for ghrelin receptor expression. The mouse is made such that the action of Cre-recombinase will reactivate ghrelin receptor expression. We will cross our mice to AgRP-Cre mice, which will result in mice with ghrelin receptor expression only in NPY/AgRP neurons. We will assess the responses to acute ghrelin administration and sensitivity to diet induced obesity in wild type, null, and AgRP reactivated mice. These studies will substantially expand our knowledge of the neural circuits engaged by ghrelin.